The present invention relates to a surface acoustic wave filter, and more particularly to an internal reflection type unidirectional surface acoustic wave filter having floating electrodes.
As means for converting a transversal surface acoustic wave filter to a low-loss version, there has hitherto been proposed a unidirectional surface acoustic wave filter. Japanese Patent Publication (b2) No. 20929 of 1991 shows that unidirectivity can be obtained by inserting an open or short-circuit type floating electrode between positive and negative finger electrodes of an interdigital transducer (IDT) and displacing the position of this floating electrode from a center position between the positive and negative finger electrodes.
For example, as illustrated in FIG. 1, when a short-circuit type floating electrode 12 is disposed between positive and negative finger electrodes 10 and 11 of the interdigital transducer in such a way as to be displaced from a center position therebetween, a surface acoustic wave is strongly excited in an arrow-indicated direction in FIG. 1, with the result that unidirectivity occurs.
In this conventional internal reflection type unidirectional surface acoustic wave filter having open or short-circuit type floating electrodes, when using electrodes weighted by an apodization or withdrawal weighted electrodes in order to suppress the outside-band sidelobe levels, a structure of finger electrodes that includes the short-circuit type floating electrodes is as illustrated in FIGS. 2 and 3.
In a unidirectional surface acoustic wave filter with an IDT finger electrode width of .lambda./12 (.lambda.: wavelength) and short-circuit type floating electrodes 12, such as that illustrated in FIG. 3, when using the withdrawal weighted electrodes, the following phenomenon occurs at portions where the positive and negative finger electrodes 10 and 11 do not cross each other, i.e., in case where adjacent finger electrodes are the same positive finger electrode 10 or negative finger electrode 11. That is, although these finger electrodes 10, 11 or floating electrodes 12 have no relevancy to the excitation of surface acoustic waves, they disturb the phase thereof by acoustic reflection thereof, with the result that the inside-passband characteristic exhibits ripples as illustrated in FIG. 4.
FIG. 4 illustrates an amplitude characteristic 41 and a group delay characteristic 42 obtained from a filter prepared by disposing the electrode structure of FIG. 3 with respect to each of input and output transducers on a quartz substrate. In this graphic diagram, the center frequency is 201 MHz (megahertz'), the thickness of the aluminium electrode is 200 nm (nanometers) and the impedance is 90 ohms at the input side and 150 ohms at the output side. The axis of abscissas represents the frequency and is graduated in 100 kHz (kilohertz') per graduation. The axis of ordinates represents the insertion loss of the amplitude characteristic and the time delay of the group delay characteristic and is graduated in 1 dB (decibel) and 1 .mu.s (microsecond), respectively, per graduation.
In the conventional internal reflection type unidirectional surface acoustic wave filter, there is the problem that when using the withdrawal or apodization weighted electrodes, ripples occur inside the passband and so the filtering characteristic deteriorates.
The unidirectional surface acoustic wave filter is theoretically discussed in a paper contributed by K. Yamanouchi and another to the ELECTRONICS LETTERS, Vol. 20, No. 24 (Nov. 22, 1984), pages 989 to 990, under the title of "NEW LOW-LOSS SAW FILTER USING INTERNAL FLOATING ELECTRODE REFLECTION TYPES OF SINGLE-PHASE UNIDIRECTIONAL TRANSDUCER" and a paper contributed by T. Kodama and three others to the IEEE 1986 ULTRASONICS SYMPOSIUM, pages 59 to 64, under the title of "DESIGN OF LOW-LOSS SAW FILTERS EMPLOYING DISTRIBUTED ACOUSTIC REFLECTION TRANSDUCERS".